This invention relates generally to video display terminals and more particularly to apparatus for recording in memory for later reproduction all data displayed on the scan line raster of a predetermined video frame.
When using a video display terminal (VDT) in a computer system, a permanent record of a frame of data as displayed on the face of the cathode ray tube is often desirable. The exhibited data are in the form of binary bits controlling the unblanking of the electron beam that is being repeatedly scanned across the face of the tube. Each display frame is thus formed of a matrix of fluorescent dots with each dot representing the presence of a binary bit. Source data are generally provided from a random access memory to permit regeneration of the frame as long as required.
Hard copy printers, because of their relative slowness, require a form of buffer storage into which the binary data are read from the source and temporarily retained until the printer can complete the job much later. The frame data can be obtained either from a memory for the display device or directly from the video input to the display from a remote source.
Buffer storage arrangements for enabling printers to produce frame copy in the past have used several techniques to capture the bit data for the particular frame. One technique has been to temporarily store character codes that are transmitted from the display to a character generator; these codes also select the desired character for printing from the printer's own character generator. Another technique has been to capture the bits of only a single scan line, record them, then repeat the process for each scan line in succession to compose the frame. Yet another technique has been to capture a predetermined segment or portion of each of the scan lines and print the frame transversely of its original orientation.
This is the technique described in the aforementioned patent application Ser. No. 271,911.
The first method has the advantage of storing only character codes and tying up the display unit for the minimum time. However, not all data points on the screen are addressable since only data lines and characters within a line can be recorded. This type of recording is unable to provide copies of diagrams or charts which are frequently necessary. The second technique requires minimum buffer storage, since it stores only the bits in a single scan line, but the printer must record each line before the next line of bits can be stored. Thus the use of a display unit is delayed extensively until all lines are sequentially printed. An example of this technique is the system shown in U.S. Pat. No. 4,158,203. Further, the printing device must be equal in length to a full scan line, usually having several hundred elements. With the third alternative, as mentioned, the printed copy is usually rotated by 90 degrees, and the VDT is connected to the printer for a moderate amount of time since only the segments of each scan line are printed during a single excursion of the print head. Both of the latter techniques have the advantage of placing in buffer storage all data points addressable on the face of the VDT, thereby enabling the recording of diagrams and charts.
The foregoing types of display copying, however, each require either relatively lengthy connection time between the VDT and printer or cannot capture all data points displayed on the screen.